Dirk truncate event categories in rh menu

This pull is related to the issue "Align titles of events in layer menu":
https://github.com/nens/lizard-nxt/issues/264

This pull is x-referenced in: https://github.com/nens/lizard-nxt/pull/270

todo:

- [x] create ng filter
- [x] use this filter in base.html
- [ ] show full name for Event category on mouseover